Low profile mount for plural lower electrode metal oxide varistor package and method

ABSTRACT

A low profile mount for a disc varistor. A thermally sensitive switch is provided both for single and multiple electrode embodiments. The switch may be placed in a shorting circuit and include a spring biased conductor prevented from closure by a heat sensitive element which softens in responsive to excessive heat. The varistor may be fused to prevent excessive current from a short circuited but not open circuited varistor. Methods are also provided.

This application is a division of U.S. patent application Ser. No.09/401,497, filed Sep. 22, 1999.

BACKGROUND OF THE INVENTION

This invention relates to a surface mount for an electrical componentand more particularly to a surface mount for a metal oxide varistor.

Surface mounting of components on printed circuit boards is common andit is desirable that the mounts be low in profile so that the distancebetween boards can be minimized and the size of the electronic chassisthus reduced. Where the electrical component is a thin element such as adisc with electrodes on both flat sides, low profile mounts areparticularly desirable.

It is also desirable that the number of components required for mountingbe minimized because of the area on the circuit board each componentoccupies. Thus it is highly desirable that the mount be compatible withcomponents having multiple electrodes.

The thermal characteristics of the electrical components is generally aconcern and it is desirable that the surface mount be compatible with,or integrated with one or more thermally sensitive switches. The thermalcharacteristics for many electrical circuit components such as varistorsare particularly important where the component is coated with epoxy. Inthe event of a sustained overvoltage condition across the component, thecurrent through the component can cause the temperature to increasesufficiently to cause the epoxy coating to smoke or ignite. Similarly,the epoxy coating can smoke or ignite in the event the component issubjected to a significant current pulse such that the componentinternally shorts but does not destruct, i.e., does not open-circuit.

The current solution to this overheating problem is to wire in serieswith the component a thermal cutout device in sufficiently closephysical proximity to respond to the heat in the component to shortcircuit and thereby disconnect the component. This solution requires theaddition of extra components and increases the cost and complexity ofthe circuit. Since great care must be taken to keep the thermal cutoutdevice in close physical proximity to the component to preserve theresponsiveness without overreacting, interference may occur with anysurface mount.

Additionally, the “thermal connection” between the component and thethermal cutout device may be difficult to optimize in a surface mount.

Finally, low temperature thermal cutout devices may not be able tosurvive the soldering operation required to attach the component to aprinted circuit board or the like, and a surface mount to which thecomponent may be mounted without soldering is highly desirable.

Many of the above problems are exacerbated where the electricalcomponent is a disc varistor. Disc varistors are well known andgenerally comprise a thin disc of a metal oxide or other voltagevariable resistive material with an electrode on opposite flat sides ofthe varistor material. Known surface mounts for the smaller sizes, e.g.,7-10 mm, generally include a molded plastic body into which springelectrodes exiting the sides of the body are internally exposed inposition to be contacted by the electrodes of the disc varistor wheninserted therebetween from the front of the body.

One example of such prior art mount is illustrated in FIGS. 1 and 2where the disc varistor 18 comprises a disc of varistor material 20 withsurface electrodes 22 and 24 on the flat sides thereof. The varistor 18may be inserted into the front opening of a plastic body 26 whichcarries a top contact 28 and a bottom contact 30 conveniently springbiased toward each other and separated by the insertion of the discvaristor. The ends of the contacts 32, 34 may be used to electricallyconnect the circuit to other components on the printed circuit board onwhich the body 26 may be mounted. Such mounts are expensive tomanufacture and are generally higher in profile than desired. Moreover,they do not provide for multiple electrodes, for temperature protectionor for a fuse.

Accordingly, it is an object of the present invention to obviate many ofthe above problems and to provide a novel surface mount and method foran electrical component on a printed circuit board.

It is another object of the present invention to provide a novel surfacemount and method in which the mount is low profile.

It is still another object of the present invention to provide a novelsurface mount and method in which the component may be connected withoutsoldering.

It is yet another object of the present invention to provide a novelsurface mount and method which is readily adapted for multipleelectrical connections.

It is yet still another object of the present invention to provide anovel surface mount and method which is compatible with a thermalswitch.

It is a further object of the present invention to provide a novelsurface mount which is inexpensive and simple in construction.

It is yet a further object of the present invention to provide a novelsurface mount and method which short circuit protection is available.

It is still a further object of the present invention to provide a novelsurface mount and method in which the mount may be coated with amoisture barrier and/or have edge passivation.

These and many other objects and advantages of the present inventionwill be readily apparent to one skilled in the art to which theinvention pertains from a perusal of the claims, the appended drawings,and the following detailed description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a pictorial view of a prior art surface mount package for avaristor disc.

FIG. 2 is a section in elevation taken through lines 2—2 of FIG. 1.

FIG. 3 is a side view in elevation of one embodiment of the surfacemount of the present invention.

FIG. 3A is an exploded pictorial view of the mount of FIG. 3.

FIG. 4 is a side view in elevation of a second embodiment of the surfacemount of the present invention.

FIG. 5 is a side view in elevation of the embodiment of FIG. 4 with athermal switch in the open position.

FIG. 6 is a schematic circuit diagram of the varistor and switch ofFIGS. 3 and 4 with a fuse in series with the varistor.

FIG. 7 a side view in elevation of a third embodiment of the surfacemount of the present invention adapted for two electrodes on the uppersurface of the varistor.

FIG. 8 is a top plan view of a two component electrode compatible withthe mount of FIG. 7.

FIG. 9 is a schematic circuit diagram of the varistor of FIG. 8.

FIG. 10 is a side view in elevation of a fourth embodiment of thesurface mount of the present invention adapted for two electrodes on thelower surface of the varistor.

DESCRIPTION OF PREFERRED EMBODIMENTS

With reference to FIGS. 3 and 3A, a preferred embodiment of the mount ofthe present invention includes a molded plastic body 36 with a centralcircular depression dimensioned to receive a disc varistor 18 (varistormaterial 20 and electrodes 22 and 24) to a depth of about one-half thethickness thereof. Centrally located within the depression is a seconddepression adapted to receive a circular contact 38 which exits the body36 to provide a first lead 40. The contact 38 desirably extends upwardlyabove the bottom of the first depression in order to insure contact withthe bottom electrode 24 of the varistor 18.

The body 38 is desirably molded with a support 42 for the second contact44 which may be made from an electrically conductive spring wire so thatit is spring biased downwardly into contact with the upper electrode 22of the disc varistor 18. The contact extends through the support 42 toprovide a second electrical lead 46 of the mount.

As shown in FIG. 4, the leads 40 and 46 may extend downwardly so as topenetrate through a printed circuit board for electrical connection toan electrical circuit on the lower side thereof.

As described more fully in applicant's copending application Ser. No.09/665,283 filed concurrently herewith for “Thermally Protected MetalOxide Varistor And Method”, the disclosure of which is herebyincorporated herein by reference, thermal protection may be provided bya third contact 50 which may be constructed in the same manner as thecontact 44 to provide a third lead 52, but which includes a thermalelement 54 covering at least a portion of the contact 50 to preventcontact with the upper electrode 22 of the varistor 18. The thermalelement 54 may be any suitable conventional insulator responsive toexcessive heat in the varistor to either physically dissipate or to loseits insulative characteristics. An example of such a thermally reactiveinsulator is benzanilide. Benzanilide is generally rigid up to itsmelting temperature, which is generally in the range of 150-200° C. andis preferentially 163° C. Benzanilide adheres well and breaks down atits melting temperature to a viscosity that is water-like. It has beenfound desirable for the body 36 to include a second support 56 at theend of the mount opposite to the support 42 to lend rigidity to thecontact 50.

The circuit including the third contact 50 is shown in FIG. 6 where ashorting circuit is shown with the leads 52 and 40 electricallyconnected. Under normal operating conditions, current flows through thevaristor 18 via the leads 40 and 46. An undesirable thermal condition inthe varistor 18 caused by a excessive current through the varistor froma sustained overvoltage condition across the varistor, or by a highcurrent pulse through the varistor which causes an internal short in thevaristor but does not cause the varistor to destruct, i.e., to opencircuit. In the event of such an undesirable thermal, the effectivedisappearance of the insulating means 54 in response thereto permitscontact between the electrode 50 and the upper electrode 22 of thevaristor and effects a short circuit across the varistor 18.

While the spring wire contact has been found desirable, it is to benoted that other biasing means may be adequate, e.g., a discrete springfor the electrical contact or a magnet. The shape of the varistor is nota part of this invention, and it may be square or other forms as desiredso long as the depression in the body of the mount is compatible.

However, the mount of the present invention finds particular utilitywhere the disc varistor electrodes are segmented as shown, e.g., in FIG.8. With reference to FIG. 8, one of the electrodes, e.g.,the upperelectrode 22, may be segmented into two, three or four parts, eachforming a varistor element with the single lower electrode 24.

As shown in FIG. 7, an embodiment with two segments on the upperelectrode may use two spring wire contacts 60, 62 connected respectivelyto leads 64, 66 to form the circuit shown in FIG. 9. As shown in FIG.10, the identical circuit may be formed using the two segment varistorof FIG. 8 with the segmented electrode facing downwardly and a singlecontact 44 in contact with the single, now upper, electrode 24. Thisembodiment requires that the body 36 include not one central contact 38as illustrated in FIG. 3A, but two spaced contacts 38A and 38B as shownin FIG. 10.

While preferred embodiments of the present invention have beendescribed, it is to be understood that the embodiments described areillustrative only and that the scope of the invention is to be definedsolely by the appended claims when accorded a full range of equivalence,many variations and modifications naturally occurring to those of skillin the art from a perusal hereof.

What is claimed is:
 1. A low profile mount for a generally planarelectrical component having an exposed upper electrode on one sidethereof and a plurality of exposed lower electrodes on the other sidecomprising: a insulative body with an upwardly open receptacle adaptedto receive an electrical component; a plurality of spaced lower contactscarried by said body within said receptacle, each of said lower contactsbeing electrically isolated from all other contacts in said plurality ofcontacts and each being in position to contact one of said plurality oflower electrodes when a component is received in said receptacle; anupper contact carried by said body above said receptacle in position tocontact the upper electrode of a component when received therein, saidupper contact being electrically isolated from said lower contacts andbeing downwardly biased toward said receptacle.
 2. The mount of claim 1wherein the component is a varistor.
 3. The mount of claim 1 whereinsaid plurality is four.
 4. A method of providing plural electrical pathsthrough a generally planar electrical component having an exposed upperelectrode on one side thereof and a plurality of exposed lowerelectrodes on the other side comprising the steps of: (a) disposing theelectrical component in an insulative body with an upwardly openreceptacle adapted to receive the electrical component; (b) providing aplurality of spaced lower contacts carried by the body within thereceptacle, each of the plurality of lower contacts being in position tocontact one of the plurality of lower electrodes of the component whenthe component is received in the receptacle, each of the spacedplurality of lower contacts being electrically isolated from all othersin the plurality of lower contacts; and (c) providing an upper contactcarried by the body above the receptacle in position to contact theupper electrode of the component when received in the receptacle, theupper contact being electrically isolated from the lower plurality ofcontacts and being downwardly biased toward the receptacle, so thatplural electrical paths through said electrical component are therebyprovided.
 5. The method of claim 4 wherein said plurality of spacedlower contacts is four.
 6. The method of claim 4 wherein the componentis a varistor.
 7. A flat mount for a disc shaped electrical componenthaving a planar exposed upper electrode on one side thereof and aplurality of exposed generally coplanar lower electrodes on the otherside comprising: a insulative body with an upwardly open cylindricalreceptacle adapted to received a disc shaped electrical component; aplurality of spaced apart generally coplanar lower contacts carried bysaid body at the bottom of said receptacle radially inward of thecircumference thereof in position to contact one of said plurality oflower electrodes when a component is received in said receptacle, eachof the plurality of lower contacts being electrically isolated from allother of the plurality of lower contacts; an upper contact carried bysaid body above said receptacle in position to contact the upperelectrode of a component when received in the receptacle, said uppercontact extending upwardly through said body radially outward of saidreceptacle, and being downwardly biased toward said receptacle.